1. Field of the Invention
The present invention relates to a process for producing acetic acid from ethylene and oxygen by one-stage catalytic reaction.
2. Description of the Related Art
Acetic acid has heretofore been practically produced by a process comprising oxidizing acetaldehyde, a process comprising reacting methanol with carbon monoxide and a process comprising oxidizing lower paraffin and the like.
The process comprising oxidizing acetaldehyde is a process of two-stage oxidation, namely, oxidation of ethylene to form acetaldehyde and oxidation thereof to form acetic acid. Since the Pd ion Contributing to oxidation of ethylene in this process cannot oxidize acetaldehyde thus produced, the catalysts used in the two stages differ from each other. Direct synthesis of acetic acid by this process is therefore difficult. The process comprising carbonylation of methanol has a disadvantage that the cost of Rhodium, the catalyst used in this process, is extremely high. On the other hand, the process comprising oxidizing lower hydrocarbon synthesizes acetic acid by one stage. However, the reaction conditions are comparatively strict, and as a result many by-products are formed. Accordingly, the process has problems of improving the reaction selectivity and the yield. Furthermore, the processes mentioned above are all carried out by liquid phase homogeneous reactions, and therefore require complicated operations to recover catalysts, separate products, and perform other procedures.
Many catalysts have been proposed for processes of producing acetic acid from ethylene by one-stage oxidation. For example, there is a disclosure of a process wherein Pd metal-phosphoric acid or a sulfur-containing catalyst modifying agent is used (Japanese Unexamined Patent Publication (Kokai) Nos. 47-13221 and 51-29425). Moreover, there is a disclosure of a process in which the use of a catalyst, a Pd salt of a certain type of heteropoly-acid, is effective (Japanese Unexamined Patent Publication (Kokai) No. 54-57488). It is particularly industrially important in carrying out reactions using these catalysts to enhance the activity thereof, reduce the deterioration of age thereof as much as possible, and use catalysts exhibiting good selectivity of acetic acid. However, it is difficult to conclude that those catalysts which have heretofore been proposed exhibit satisfactory properties when used in the production of acetic acid on a industrial scale.
Further, there is proposed a gas phase one stage oxidation process in which a catalyst consisting of a three-component oxygen compound prepared by using a heteropoly-acid is used (Japanese Examined Patent Publication (Kokoku) No. 46-6763). The heteropoly-acid is used as a source for providing elements necessary for forming the three-component oxygen compound and, thus, is converted thereto as a result of sintering during the preparation process of the catalyst.